Recently, interest in energy storage technology is rising. As its application is spreading into mobile phones, camcorders, notebook computers, PCs and electric vehicles, research and development therefor are being intensive more and more. In this regard, electrochemical devices are the most highlighted field, and particularly, development of rechargeable secondary batteries is being the focus of attention.
Among the currently used secondary batteries, a lithium secondary battery, developed in early 1990's, is a compact, light-weight and high capacity battery, and has been widely used as a power source of portable devices since its appearance in 1991's. The lithium secondary battery is being highlighted due to its higher drive voltage and energy density than those of conventional batteries using aqueous electrolytes (such as Ni-MH batteries, Ni—Cd batteries and Pb-sulfuric acid batteries). Particularly, researches about a power source for an electric vehicle, wherein an internal-combustion engine and a lithium secondary battery are hybridized, are actively proceeding in America, Japan, Europe and the like.
However, in view of energy density, the use of a lithium ion battery as a large battery for an electric vehicle has been considered, but in view of stability, a Ni-MH battery has been being used so far. In the lithium ion battery for an electric vehicle, the biggest challenge is high cost and safety. Particularly, if an over-charged battery is heated at 200 to 270° C., the structure of the commercially used positive electrode active materials such as LiCoO2 and LiNiO2 are rapidly changed. And, due to its structural change, the crystal structure becomes instable by the reaction releasing oxygen in a lattice, and delithiation during charging, and therefore, thermal stability also becomes very poor.
For improving this, various methods, for example, a method substituting a part of Ni (nickel) into a transition metal to shift the temperature of heat generation to a little higher temperature or to inhibit rapid heat generation are being attempted. A material substituting a part of Ni into Co (cobalt), i.e., LiNi1−xCoxO2 (x=0.1-0.3), showed excellent charging/discharging characteristics and lifespan, but did not solve the stability problem. Further, Li—Ni—Mn-based composite oxide, wherein a part of Ni is substituted with Mn having excellent thermal stability, or Li—Ni—Mn—Co-based composite oxide, wherein a part of Ni is substituted with Mn and Co, and technologies for preparing thereof are well known. Further, Japanese Patent No. 2000-227858 disclosed a positive electrode active material prepared by not the technology partly substituting transition metal to LiNiO2 or LiMnO2, but a novel technology uniformly distributing Mn and Ni compounds in atomic level to obtain a solid solution.
According to European Patent No. 0918041 or U.S. Pat. No. 6,040,090 about compositions of Li—Ni—Mn—Co-based composite oxide, wherein Ni is substituted with Mn and Co, LiNi1−xCoxMnyO2 (0<y≤0.3) showed better thermal stability than the existing material comprising only Ni and Co, but it could not solve the problem of thermal stability of the Ni-based compound yet.
In order to solve the problem, methods changing the surface composition of the positive electrode active material adjacent to electrolyte are applied, and one of these methods is a surface-coating method. Generally, it is known that the amount of coating is very small of 1 to 2% by weight or less based on a positive electrode active material, and the coating layer forms a very thin membrane layer of about several nanometers to prevent side reaction with electrolyte. Or, sometimes, when the heat-treating temperature after coating is too high, a solid solution is formed on the surface of the powder particles and the metal composition is different with the composition inside the particle. In this case, the thickness of the surface layer bound to the coating material is several nanometers or less, and there is a dramatic difference between the coating layer and the particle bulk. Therefore, after long-term use of hundreds of cycles, the effect becomes lower. Further, the effect is behalved by incomplete coating, namely, nonuniform distribution of the coating layer on the surface.
In order to solve the problem, Korean Patent Application No. 10-2005-7007548 disclosed a lithium transition metal oxide having the concentration-gradient of metal. This method could synthesize an internal layer and an external layer with different metal composition, but the metal composition was not gradually and continuously changed in the synthesized positive electrode active material. Through a heat-treating process, gradual concentration-gradient of the metal composition could be obtained, but difference in the concentration-gradient was hardly formed at high heat-treating temperature of 850° C. or more due to thermal diffusion of metal ions. Further, the powder synthesized by this invention had low tapped density because the powder did not used ammonia as a chelating agent. Therefore, it was not enough to be used as a positive electrode active material for a lithium secondary battery. Further, in this method, when the lithium transition metal oxide was used as an internal material, reproducibility was reduced due to difficulty in controlling the amount of lithium on the outer layer.
Korean Patent Application No. 10-2004-0118280 suggested a bi-layer structure having a core-shell structure. This patent disclosed a material having high capacity and excellent thermal stability prepared by: combining a positive electrode composition having high capacity to a core; and a positive electrode composition having excellent thermal stability to an outer shell, by using a CSTR (Continuous Stirring Tank Reactor). However, it was difficult to form a layer having continuous concentration distribution between two interfaces, where the internal core and the outer shell meet, due to diffusion of metal elements on the interfaces. Namely, an excellent positive electrode active material, which satisfies both high efficiency and lifespan, could not be obtained.
In order to solve the problems, Korean Patent Application No. 10-2006-0059784 suggested a structure having continuous concentration-gradient of the metal composition from the contacting interface of an inner core and an outer bulk unit to the contacting interface of the outer bulk unit and an outer shell. The positive electrode active material having the said structure could satisfy both high efficiency and lifespan, but mass production of the positive electrode active material having the said structure was difficult. In this patent, a positive electrode active material having the said structure was generally prepared using a CSTR due to easy adjustment of compositional ratio. In case of the CSTR, the temperature, concentration and residence time of all reactants in the reactor were same. However, practically, there was difference of reacting condition such as temperature and concentration of each part in the reactor in an initial step supplying reactants, and until the reacting condition such as temperature and concentration in the reactor becomes same, i.e., theoretical state, all raw materials used as the initial reactant and initial product were discarded, and consequently, the yield of the product compared with the supplied raw material was low. Further, when a positive electrode active material precursor and a positive electrode active material were prepared using the conventional CSTR, supplying the raw material and discharging the product were simultaneously conducted continuously, there could be a variation between the residence time and the reaction time in the reactor of the positive electrode active material precursor and the positive electrode active material produced in the reactor, and consequently, there was a problem of non-uniformity in the size and constituent of the produced particle.